Polymer composition with improved stress whitening resistance

ABSTRACT

A polymer composition is provided with improved stress whitening resistance, having at least one thermoplastic polymer material and a dielectric liquid. A process for preparing the polymer composition, a cable having at least one electrically insulating layer obtained from the polymer composition, and a process for preparing the cable are also provided.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/062,416, filed on Jun. 14, 2018, which in turn is a National Phase ofPCT Application No. PCT/FR2016/053480 filed on Dec. 15, 2016, which inturn claims the benefit of priority from French Patent Application No.15 62790, filed on Dec. 18, 2015.

FIELD OF THE INVENTION

The invention relates to a polymer composition with improved stresswhitening resistance, comprising at least one thermoplastic polymermaterial and a dielectric liquid, to a process for preparing saidpolymer composition, to a cable comprising at least one electricallyinsulating layer obtained from said polymer composition, and to aprocess for preparing said cable.

The invention applies typically, but not exclusively, to electric cablesintended for power transmission, especially medium-voltage power cables(especially from 6 to 45-60 kV) or high-voltage power cables (especiallyabove 60 kV, and which may be up to 400 kV), whether for DC or ACcurrent, in the fields of submarine or terrestrial electricitytransmission.

FIELD OF THE INVENTION

A medium-voltage or high-voltage power transmission cable generallycomprises, from the interior to the exterior:

-   -   an elongated electrically conducting element, especially made of        copper or aluminium;    -   a semi-conducting internal layer surrounding said elongated        electrically conducting element;    -   an electrically insulating layer surrounding said        semi-conducting internal layer;    -   a semi-conducting external layer surrounding said insulating        layer; and    -   optionally, an electrically insulating protective sheath        surrounding said semi-conducting external layer.

In this type of cable, the electrically insulating layer may be apolymer layer based on a crosslinked polyethylene (XLPE). Thecrosslinking is generally performed during the step of extrusion of thepolymer composition around the elongated electrically conductingelement. The use of a crosslinked polyolefin provides a cable which canfunction at a temperature above 70° C., or even equal to 90° C. However,several problems are encountered. Firstly, crosslinked materials cannotbe recycled. Secondly, crosslinking (vulcanization) to produce ahomogeneous layer requires specific reaction conditions (e.g. in termsof duration and temperature) which reduce the rate of manufacture of thecable and increase its production cost. Finally, crosslinking mayoccasionally start prematurely in the extruder and/or the extruder head,leading to degradation of the quality of the layer obtained, especiallyof its dielectric properties.

Alternatives have thus been proposed, such as a thermoplastic layer oflow-density polyethylene (LDPE) or of high-density polyethylene (HDPE).However, a cable comprising such an electrically insulating layer cannotfunction at a temperature above about 70° C. for an LDPE thermoplasticlayer and above 80° C. for an HDPE thermoplastic layer, leading to alimitation of the power that can be transmitted in said cable and of themanufacturing methods.

Electrically insulating layers based on polypropylene (e.g. heterophasicpropylene copolymer) have been proposed. However, these layers withstandstress whitening with difficulty. Thus, when these polymers are bentand/or receive impacts, the bent area and/or the area which has receivedan impact becomes opaque and whitish even when the polymer is coloured.This phenomenon may occur, for example, during rolling-up at the time ofinstallation of a cable. Now, stress whitening is not desired since itincreases the risk of cracking and/or leads to the formation of defectsin the layer, bringing about degradation of the electrical properties ofsaid layer.

It is known practice to improve the stress whitening resistance byadding polyethylene. In particular, EP 1 510 547 A1 describes a polymercomposition comprising from 70% to 98% by mass of a heterophasicpropylene copolymer comprising one phase based on a propylenehomopolymer and one phase based on a copolymer of propylene and ofethylene and/or of one or more C₄-C₁₂ α-olefins and from 2% to 30% bymass of an ethylene polymer. However, this polymer composition is notoptimized in terms of stress whitening resistance and of dielectricbreakdown strength to be used in an electrically insulating layer of amedium-voltage or high-voltage cable.

OBJECTS AND SUMMARY

Thus, the aim of the present invention is to overcome the drawbacks ofthe prior art and to provide an economical polymer composition usingrecyclable materials, which can give an electrically insulating layerthat has improved mechanical properties, especially in terms of stresswhitening resistance, while at the same time ensuring good dielectricproperties, especially in terms of dielectric breakdown strength.

The aim of the present invention is also to provide an economical cable,in particular a medium-voltage or high-voltage cable, which can functionat temperatures above 70° C. and which has improved mechanicalproperties, especially in terms of stress whitening resistance, while atthe same time ensuring good dielectric properties, especially in termsof dielectric breakdown strength.

The aims are achieved by the invention which will be describedhereinbelow.

A first subject of the invention is a polymer composition comprising atleast one polypropylene-based thermoplastic polymer material and adielectric liquid, characterized in that the dielectric liquid comprisesat least one compound corresponding to formula (I) below:

R¹-A-R²  (I)

in which R¹ and R², identical or different, are unsubstituted arylgroups and the element A represents a single bond or an alkylene group.

The aryl group may comprise one or more fused or non-fused, andpreferably non-fused, aromatic rings.

The aryl group may comprise from 5 to 20 carbon atoms and preferablyfrom 6 to 12 carbon atoms.

Each aromatic ring may comprise one or more heteroatoms such as anitrogen atom, a sulfur atom or an oxygen atom.

The expression “unsubstituted aryl groups” means that each of the arylgroups of the compound of formula (I) does not comprise any monovalentsubstituent(s), and especially is not substituted with one or more alkylgroups of formula C_(t)H_(2t+1) (e.g. 1≤t≤5) such as methyl groups.

The aryl groups of the compound of formula (I) are therefore notalkyl-aryl groups.

The element A may be a linear, cyclic or branched, preferably linear orcyclic and more preferably linear alkylene group.

In particular, the element A may be an alkylene group containing from 1to 10 carbon atoms and preferably from 1 to 5 carbon atoms.

Preferably, the alkylene group is a group —(CH₂)_(n)— with 1≤n≤10; agroup —(CHR)_(n′)— with 1≤n′1≤5 and R being an alkyl group, preferablycontaining from 1 to 5 carbon atoms; a statistical group—(CHR)_(p)—(CH₂)_(m)— (i.e. comprising m —CH₂— and p —CHR—), with1≤p+m≤9, and R being an alkyl group, preferably containing from 1 to 5carbon atoms; or a statistical group —(CHR)_(p1)—(CH₂)_(m′)—(CHR′)_(p2)—(i.e. comprising m —CH₂—, p₁-CHR— and p₂-CHR′—), with 1≤p₁+m′+p₂≤8, andR and R′ being different alkyl groups, each preferably containing from 1to 5 carbon atoms, preferably with 1≤p≤4, 1≤m≤8, 1≤p₁≤3, 1≤m′≤6 and1≤p₂≤3.

In the present invention, a statistical group means that the radicalswhich constitute it (e.g. —CH₂—, —CHR— and/or —CHR′—) may be randomlypositioned in the element A.

R (or, respectively, R′) may be a methyl, ethyl, propyl or isopropylgroup.

When the element A (connecting the aryl groups) is a branched alkylenegroup (e.g. presence of at least either of the groups R and R′), it mayalso be connected by branching (e.g. via R or R′) to R¹ and/or R².

The aryl group is preferably a phenyl group, a naphthyl group or apyridyl group, and more preferably a phenyl group.

According to a first variant of the invention, at least one of saidgroups R¹ or R² of the compound of formula (I) is a phenyl group.

According to a second variant of the invention, the two aryl groups eachcomprise a phenyl group.

According to a third variant, the groups R¹ and R² of the compound offormula (I) are each phenyl groups.

According to a particularly preferred embodiment of the invention, thecompound of formula (I) may be 1,2-diphenylethane (i.e. R¹=R²=phenyl andA=—CH₂—CH₂—), 1,1-diphenylethane (i.e. R¹=R²=phenyl and A=—CH(CH₃)—,diphenylmethane (i.e. R¹=R²=phenyl and A=—CH₂—) or1,2,3,4-tetrahydro(1-phenylethyl)naphthalene (i.e. R¹=R²=phenyl andA=—CHCH₃—CH—(CH₂)₃—).

The dielectric liquid is generally liquid at about 20-25° C.

The dielectric liquid may comprise at least 50% by mass approximately ofat least one compound of formula (I), and preferably at least 80% bymass approximately of at least one compound of formula (I), relative tothe total mass of the dielectric liquid. By means of this minimum amountof compound(s) (I), the stress whitening resistance is improved.

Preferably, the dielectric liquid is constituted solely of a compound offormula (I) or of several compounds of formula (I).

The ratio of the number of aromatic carbon atoms to the total number ofcarbon atoms in the dielectric liquid may be greater than or equal toabout 0.6; and preferably greater than about 0.6.

The ratio of the number of aromatic carbon atoms to the total number ofcarbon atoms in the dielectric liquid may be determined according tostandard ASTM D3238 or on the basis of the chemical formula.

The presence of a compound of formula (I) in the polymer compositionmakes it possible to improve the stress whitening resistance of theelectrically insulating layer of an electric cable, while at the sametime ensuring good dielectric breakdown strength. Moreover, the presenceof a polypropylene-based thermoplastic polymer material makes itpossible to increase the operating temperature of the cable to 90°C.-110° C.

Preferably, the dielectric liquid has a boiling point of greater thanabout 250° C.

Thus, the dielectric liquid of the polymer composition of the inventionmay be manipulated without risk at room temperature (sparingly volatile)and at the temperatures required by the process for forming theelectrically insulating layer (e.g. extrusion), while at the same timeensuring the formation of a homogeneous intimate mixture with thepolymer material of the polymer composition of the invention.

The polypropylene-based thermoplastic polymer material may comprise atleast one homopolymer or one copolymer of propylene (P₁), and optionallyat least one homopolymer or one copolymer of α-olefin (P₂).

The combination of polymers P₁ and P₂ makes it possible to obtain athermoplastic polymer material with good mechanical properties,especially in terms of elastic modulus, and good electrical properties.

In particular, the propylene copolymer P₁ may be a statistical propylenecopolymer.

Examples of propylene copolymers P₁ that may be mentioned includecopolymers of propylene and of olefin, the olefin being chosenespecially from ethylene and an α-olefin other than propylene.

The α-olefin other than propylene may correspond to the formulaCH₂═CH—R³ in which R³ is a linear or branched alkyl group containingfrom 2 to 10 carbon atoms, and may be chosen especially from thefollowing olefins: 1-butene, 1-pentene; 4-methyl-1-pentene, 1-hexene,1-octene, 1-decene, 1-dodecene, and a mixture thereof.

The olefin of the copolymer of propylene and olefin preferablyrepresents not more than 15 mol % and more preferably not more than 10mol % of the copolymer.

The copolymers of propylene and ethylene are preferred as propylenecopolymer P₁.

The propylene copolymer P₁ preferably has an elastic modulus rangingfrom about 600 to 1200 MPa.

The propylene homopolymer P₁ preferably has an elastic modulus rangingfrom about 1250 to 1600 MPa.

The propylene homopolymer or copolymer P₁ may have a melting point ofgreater than about 130° C., preferably greater than about 140° C., andmore preferably ranging from about 140 to 165° C.

In particular, the propylene homopolymer P₁ may have a melting point ofabout 165° C. and the propylene copolymer P₁ may have a melting pointranging from about 140 to 150° C.

The propylene homopolymer or copolymer P₁ may have a heat of fusionranging from about 30 to 100 J/g.

In particular, the propylene homopolymer P₁ may have a heat of fusionranging from about 80 to 90 J/g and the propylene copolymer P₁ may havea heat of fusion ranging from about 30 to 70 J/g.

The propylene homopolymer or copolymer P₁ may have a melt flow indexranging from 0.5 to 3 g/10 minutes, measured at about 230° C. with anapproximately 2.16 kg load according to standard ASTM D1238-00.

According to a preferred embodiment of the invention, the propylenehomopolymer or copolymer P₁ represents from about 40% to 70% by mass ofthe polypropylene-based thermoplastic polymer material.

The α-olefin of the α-olefin homopolymer or copolymer P₂ may correspondto the formula CH₂═CH—R⁴ in which R⁴ is a hydrogen atom or a linear orbranched alkyl group containing from 1 to 12 carbon atoms, and may bechosen especially from the following olefins: ethylene, propylene,1-butene, isobutylene, 1-pentene, 4-methyl-1-pentene, 1-hexene,1-octene, 1-decene, 1-dodecene, and a mixture thereof.

The α-olefin propylene, 1-hexene or 1-octene is preferred.

The α-olefin homopolymer or copolymer P₂ may be a heterophasic copolymercomprising a thermoplastic phase of propylene type and a thermoplasticelastomer phase of the type copolymer of ethylene and of an α-olefin, apolyethylene or a mixture thereof.

The thermoplastic elastomer phase of the heterophasic copolymer mayrepresent at least 20% by mass approximately, and preferably at least45% by mass approximately, relative to the total mass of theheterophasic copolymer.

The α-olefin of the thermoplastic elastomer phase of the heterophasiccopolymer may be propylene.

The polyethylene may be a linear low-density polyethylene. In thepresent invention, the term “low-density polyethylene” means a linearpolyethylene with a density ranging from about 0.91 to 0.925.

According to a preferred embodiment of the invention, the α-olefinhomopolymer or copolymer P₂ represents from about 30% to 60% by mass ofthe polypropylene-based thermoplastic polymer material.

The thermoplastic polymer material of the polymer composition of theinvention is preferably heterophasic (i.e. it comprises several phases).The presence of several phases generally originates from the mixing oftwo different polyolefins, such as a mixture of polypropylene and of acopolymer of propylene or of polyethylene.

According to a particularly preferred embodiment of the invention, thethermoplastic polymer material comprises a copolymer of propylene andethylene [as propylene homopolymer or copolymer P₁] and a heterophasiccopolymer comprising a thermoplastic phase of propylene type and athermoplastic elastomer phase of the type such as a copolymer ofethylene and propylene [as α-olefin homopolymer or copolymer P_(2]).

The polymer composition of the invention comprises an intimate mixtureof the dielectric liquid and of the thermoplastic polymer material (e.g.it forms a homogeneous phase).

The mass concentration of the dielectric liquid in the polymercomposition is preferably less than or equal to the saturation massconcentration of said dielectric liquid in the thermoplastic polymermaterial.

The saturation mass concentration at 20-25° C. is generally from about15% to 20%. It may be determined by the liquid absorption method. Inparticular, plates (e.g. 200 mm×200 mm×0.5 mm in size) made of thepolypropylene-based thermoplastic polymer material of the polymercomposition are prepared from the corresponding starting materials,especially by moulding. Samples of these plates are weighed (initialweight=P₀) and then immersed at about 20° C. into the dielectric liquidof the polymer composition. The saturation mass concentration ismeasured by determining the weight change (as a percentage) of thesamples after various immersion times (e.g. 3, 6, 9, 12 and 15 days) andafter the surface thereof has been cleaned and dried (finalweight=P_(f)). The absorption of the dielectric liquid is determinedaccording to the following formula:

% absorption of dielectric liquid=[(P _(f) −P ₀)/P ₀]×100

The saturation concentration is reached when P_(f) shows a variation ofless than 1% relative to the total weight increase which corresponds toP_(f)−P₀.

According to a particular embodiment, the dielectric liquid representsfrom about 1% to 20% by mass, preferably from about 2% to 15% by massand more preferably from about 3% to 12% by mass relative to the totalmass of the polymer composition.

According to a particular embodiment, the polypropylene-basedthermoplastic polymer material represents from about 70% to 98% by mass,preferably from about 80% to 95% by mass and more preferably from about88% to 97% by mass relative to the total mass of the polymercomposition.

The polymer composition may also comprise one or more additives.

The additives are well known to those skilled in the art and may bechosen from antioxidants, UV stabilizers, copper scavengers, watertreeing inhibitors, and a mixture thereof.

The polymer composition of the invention may typically comprise fromabout 0.01% to 5% by mass and preferably from about 0.1% to 2% by massof additives relative to the total mass of the polymer composition.

More particularly, the antioxidants protect the polymer compositionagainst the thermal stresses generated during the steps formanufacturing the cable or during the functioning of the cable.

The antioxidants are preferably chosen from hindered phenols, thioesters, sulfur-based antioxidants, phosphorus-based antioxidants,antioxidants of amine type, and a mixture thereof.

Examples of hindered phenols that may be mentioned includepentaerythrityltetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (Irganox®1010), octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate(Irganox® 1076),1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene(Irganox® 1330), 4,6-bis(octylthiomethyl)-o-cresol (Irgastab® KV10),2,2′-thiobis(6-tert-butyl-4-methylphenol) (Irganox® 1081),2,2′-thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate](Irganox® 1035), 2,2′-methylenebis(6-tert-butyl-4-methylphenol),1,2-bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl)hydrazine (Irganox® MD1024) and 2,2′-oxamidobis(ethyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate).

Examples of thio esters that may be mentioned include didodecyl3,3′-thiodipropionate (Irganox® PS800), distearyl thiodipropionate(Irganox® PS802) and 4,6-bis(octylthiomethyl)-o-cresol (Irganox® 1520).

Examples of sulfur-based antioxidants that may be mentioned includedioctadecyl 3,3′-thiodipropionate and didodecyl 3,3′-thiodipropionate.

Examples of phosphorus-based antioxidants that may be mentioned includetris(2,4-di-tert-butylphenyl) phosphite (Irgafos® 168) andbis(2,4-di-tert-butylphenyl)pentaerythrityl diphosphite (Ultranox® 626).

Examples of antioxidants of amine type that may be mentioned includephenylenediamines (e.g. 1PPD or 6PPD), diphenylaminestyrenes,diphenylamines, mercaptobenzimidazoles and polymerized2,2,4-trimethyl-1,2 dihydroquinoline (TMQ).

An example of a mixture of antioxidants that may be mentioned is IrganoxB 225, which comprises an equimolar mixture of Irgafos 168 and Irganox1010 as described above.

The polymer composition is a thermoplastic polymer composition.

It is thus not crosslinkable.

In particular, the polymer composition does not comprise anycrosslinking agents, silane-type coupling agents, peroxides and/oradditives that enable crosslinking. The reason for this is that suchagents degrade the polypropylene-based thermoplastic polymer material.

The polymer composition is preferably recyclable.

A second subject of the invention is a process for preparing the polymercomposition in accordance with the first subject, characterized in thatit comprises at least one step i) of mixing a polypropylene-basedthermoplastic polymer material with a dielectric liquid as defined inthe first subject of the invention.

In particular, the mixing is performed according to the followingsubsteps:

i-a) optionally, mixing a compound of formula (I) as defined in thefirst subject of the invention with the additive(s) as defined in thefirst subject of the invention, and i-b) mixing a polypropylene-basedthermoplastic polymer material as defined in the first subject of theinvention with at least one compound of formula (I) or with the mixtureas obtained in the preceding substep i-a) if substep i-a) exists.

The polypropylene-based thermoplastic polymer material of substep i-b)is generally in the form of polymer granules, especially of granules ofat least one propylene homopolymer or copolymer P₁ and optionally of atleast one α-olefin homopolymer or copolymer P₂ as defined in the firstsubject of the invention.

The mixing of substep i-a) may be performed using any machine fordissolving the additive(s) as defined in the first subject of theinvention (especially when they are in the form of solid powders), atleast in the compound of formula (I) of the dielectric liquid.

Substep i-a) is preferably performed at a temperature ranging from about20 to 100° C., preferably from about 50 to 90° C. and more preferably ata temperature of about 70° C.

Substep i-a) generally lasts from 15 minutes to 1 hour and preferablyfrom 20 to 30 minutes.

On conclusion of substep i-a), a stable, transparent solution isobtained.

The mixing of substep i-b) may be performed by mixing the mixtureobtained in substep i-a) with the polypropylene-based thermoplasticpolymer material or the polymeric compounds which constitute it,especially using an internal mixer, especially with tangential rotors orwith gear rotors, or a continuous mixer, especially a screw orcounter-rotating twin-screw mixer or a mixer of “Buss extruder” type.

In the course of substep i-b), the polymer composition of the inventionmay be formed, especially in the form of granules.

To do this, the temperature in the mixer is chosen to be sufficient toobtain the thermoplastic polymer material in melt form. Next, thehomogeneous mixture may be granulated, via techniques that are wellknown to those skilled in the art. These granules can then feed anextruder to manufacture the cable of the invention according to aprocess as defined below.

A third subject of the invention is a cable comprising at least oneelongated electrically conducting element, and at least one electricallyinsulating layer obtained from a polymer composition as defined in thefirst subject of the invention.

The electrically insulating layer of the invention is a non-crosslinkedlayer.

The electrically insulating layer of the invention is preferably arecyclable layer.

The electrically insulating layer of the invention may be an extrudedlayer, in particular extruded via processes that are well known to thoseskilled in the art.

In the present invention, the term “electrically insulating layer” meansa layer whose electrical conductivity may be not more than 1×10⁻⁹ S/mand preferably not more than 1×10⁻⁹ S/m (Siemens per metre) (at 25° C.).

The cable of the invention more particularly relates to the field ofelectric cables functioning with direct current (DC) or alternatingcurrent (AC).

The electrically insulating layer of the invention may surround theelongated electrically conducting element.

The elongated electrically conducting element may be a mono-coreconductor, for instance a metal wire, or a multi-core conductor such asa plurality of optionally twisted metal wires.

The elongated electrically conducting element may be made of aluminium,of aluminium alloy, of copper, of copper alloy, and of a combinationthereof.

According to a preferred embodiment of the invention, the electric cablemay comprise:

-   -   a first semi-conducting layer surrounding the elongated        electrically conducting element,    -   an electrically insulating layer surrounding the first        semi-conducting layer, said electrically insulating layer being        as defined in the invention, and    -   a second semi-conducting layer surrounding the electrically        insulating layer.

In the present invention, the term “semi-conducting layer” means a layerwhose electrical conductivity may be at least 1×10⁻⁹ S/m (Siemens permetre), preferably at least 1×10⁻³ S/m, and may preferably be less than1×10³ S/m (at 25° C.).

In a particular embodiment, the first semi-conducting layer, theelectrically insulating layer and the second semi-conducting layerconstitute a three-layer insulation. In other words, the electricallyinsulating layer is in direct physical contact with the firstsemi-conducting layer, and the second semi-conducting layer is in directphysical contact with the electrically insulating layer.

The cable may also comprise an electrically insulating sheathsurrounding the second semi-conducting layer, and may be in directphysical contact therewith.

The electric cable may also comprise a metal shield surrounding thesecond semi-conducting layer. In this case, the electrically insulatingsheath surrounds said metal shield.

This metal shield may be a “wire” shield composed of an assembly ofcopper or aluminium conductors arranged around and along the secondsemi-conducting layer, a “strip” shield composed of one or moreconductive metal strips made of copper or aluminium optionally posedhelically around the second semi-conducting layer or a conductive metalstrip made of aluminium posed longitudinally around the secondsemi-conducting layer and rendered leaktight by means of the adhesive inthe overlap areas of parts of said strip, or a “leaktight” shield ofmetal tube type optionally composed of lead or lead alloy andsurrounding the second semi-conducting layer. This last type of shieldcan especially act as a barrier to moisture which has a tendency topenetrate radially into the electric cable.

The metal shield of the electric cable of the invention may comprise a“wire” shield and a “leaktight” shield or a “wire” shield and a “strip”shield.

All the types of metal shields may act as earth for the electric cableand may thus transport fault currents, for example in the case of ashort circuit in the network concerned.

Other layers, such as layers which swell in the presence of moisture,may be added between the second semi-conducting layer and the metalshield, these layers ensuring the longitudinal waterproofing of theelectric cable.

A fourth subject of the invention is a process for manufacturing anelectric cable in accordance with the third subject of the invention,characterized in that it comprises at least one step 1) of extrusion ofthe polymer composition in accordance with the first subject of theinvention around an elongated electrically conducting element, to obtainan (extruded) electrically insulating layer surrounding said elongatedelectrically conducting element.

Step 1) may be performed via techniques that are well known to thoseskilled in the art, for example using an extruder.

During step 1), the composition leaving the extruder is said to be“non-crosslinked”, the operating temperature and time in the extruderbeing optimized in consequence.

A layer extruded around said electrically conducting element, which mayor may not be in direct physical contact with said elongatedelectrically conducting element, is thus obtained at the extruderoutlet.

The process preferably does not comprise a step of crosslinking of thelayer obtained in step 1).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an electric cable according to a preferredembodiment in accordance with the invention.

For the sake of clarity, only the elements essential to theunderstanding of the invention have been represented schematically, andare not to scale.

DETAILED DESCRIPTION OF THE DRAWINGS

The medium-voltage or high-voltage power cable 1, illustrated in FIG. 1, comprises a central elongated electrically conducting element 2,especially made of copper or aluminium. The power cable 1 also comprisesseveral layers arranged successively and coaxially around this centralelongated electrically conducting element 2, namely: a firstsemi-conducting layer 3 known as the “internal semi-conducting layer”,an electrically insulating layer 4, a second semi-conducting layer 5known as the “external semi-conducting layer”, an earthing and/orprotective metal shield 6, and a protective outer sheath 7.

The electrically insulating layer 4 is a non-crosslinked extruded layer,obtained from the polymer composition according to the invention.

The semi-conducting layers 3 and 5 are thermoplastic (i.e.non-crosslinked) extruded layers.

The presence of the metal shield 6 and of the outer protective sheath 7is preferentially, but not essentially, this cable structure being wellknown per se to those skilled in the art.

EXAMPLES

1. Polymer Compositions

Table 1 below collates polymer compositions in which the amounts of thecompounds are expressed as weight percentages relative to the totalweight of the polymer composition.

Composition C1 is a comparative composition, and composition I1 is inaccordance with the invention.

TABLE 1 Polymer compositions C1 I1 I2 Propylene copolymer 50.00 50.0050.00 Linear low-density 25.00 25.00 50.00 polyethylene Heterophasicpropylene 25.00 25.00 0 copolymer 1,2,3,4-Tetrahydro 0 7.70 7.70(1-phenylethyl)naphthalene Antioxidant 0.3 0.3 0.3

The origin of the compounds in table 1 is as follows:

-   -   statistical propylene copolymer sold by the company Borealis        under the reference Bormed RB 845 MO;    -   linear low-density polyethylene sold by the company ExxonMobil        Chemicals under the reference LLDPE LL 1002 YB;    -   heterophasic copolymer sold by the company Basell Polyolefins        under the reference Adflex Q 200F;    -   dielectric liquid constituted of        1,2,3,4-tetrahydro(1-phenylethyl)naphthalene sold by the company        Dow under the reference Dowtherm RP; and    -   antioxidant sold by the company Ciba under the reference Irganox        B 225, which comprises an equimolar mixture of Irgafos 168 and        Irganox 1010.

2. Preparation of the Non-Crosslinked Layers

The compositions collated in table 1 are used as follows.

130 g of dielectric liquid and 5 g of antioxidant were mixed in a glasscontainer with stirring.

The resulting mixture was subsequently mixed with 850 g of propylenecopolymer, 425 g of linear low-density polyethylene and 425 g ofheterophasic copolymer in a container, and the resulting polymercomposition was then extruded using a twin-screw extruder (Berstorfftwin screw extruder) at a temperature of about 200° C.

A comparative layer not in accordance with the invention was prepared asdescribed above, but solely using the mixture of polymers and ofoxidant.

3. Characterization of the Non-Crosslinked Layers

The stress whitening resistance was evaluated manually by bending twolayers as prepared above from compositions C1 and I1, respectively.

The dielectric breakdown strength of the layers was measured using adevice comprising two stainless-steel hemispherical electrodes about 20mm in diameter (one electrode under tension and the other connected toearth) and a dielectric oil sold by the company Bluestar Silicones underthe reference Rhodorsil 604 V 50. By definition, the dielectricbreakdown strength is the ratio between the breakdown voltage and thethickness of the insulator. The breakdown voltage was measured at about24° C., with a humidity of about 50%, using the stepped voltage climbmethod. The applied voltage was an alternating voltage with a frequencyof about 50 Hz and the voltage step-up rate was about 1 kV/s to thepoint of breakdown. 12 measurements were taken for each non-crosslinkedlayer.

The tangent delta (tan δ) (or loss factor) of the layers as preparedabove was measured by dielectric spectroscopy using a machine sold underthe trade name Alpha-A by the company Novocontrol Technologies.

The tangent of the loss angle gives an indication regarding the energydissipated in a dielectric in the form of heat.

The tests were performed on layers with a thickness close to 0.5 mm at90° C., at a frequency of 40 to 60 Hz with a 500 V voltage adaptedaccording to the thickness of the test sample, so as to apply anelectric field of 1 kV/mm.

4. Results

The layer obtained from composition I1 showed no whitening, whereas thelayer obtained from the comparative composition C1 was revealed to besparingly resistant since a white mark at the bend appeared immediatelywhen the manual stress was applied.

The dielectric breakdown strength and loss factor results are presentedin table 2 below:

TABLE 2 Dielectric breakdown Tangent strength delta (kV/mm) at 90° C. C1129.25 7.5 × 10⁻⁵ I1 127.11 8.2 × 10⁻⁵

Consequently, the polymer compositions according to the invention havebetter properties in terms of stress whitening resistance while at thesame time ensuring good dielectric properties.

1. An electric cable comprising: at least one elongated electricallyconducting element; and at least one electrically insulating layerobtained from a polymer composition having at least onepolypropylene-based thermoplastic polymer material and a dielectricliquid, wherein the dielectric liquid comprises at least one compoundcorresponding to formula (I) below:R¹—R²  (I) in which R¹ and R², identical or different, are unsubstitutedaryl groups and the element A represents a single bond or an alkylenegroup.
 2. The electric cable according to claim 1, wherein the arylgroup comprises from 5 to 20 carbon atoms.
 3. The electric cableaccording to claim 1, wherein each of the aryl groups is not substitutedwith one or more alkyl groups of formula C_(t)H_(2t+1).
 4. The electriccable according to claim 1, wherein the element A is an alkylene groupcontaining from 1 to 10 carbon atoms.
 5. The electric cable according toclaim 4, wherein the alkylene group is a group —(CH₂)_(n)— with 1 n 10;a group —(CHR)_(n′)— with 1≤n≤5 and R being an alkyl group; astatistical group —(CHR)_(p)—(CH₂)_(m)—, with 1≤p+m≤9, and R being analkyl group; or a statistical group —(CHR)_(p1)—(CH₂)_(m′)—(CHR′)_(p2)—,with 1≤p₁+m′+p₂≤8, and R and R′ being different alkyl groups.
 6. Theelectric cable according to claim 1, wherein at least one of said groupsR¹ or R² of the compound of formula (I) is a phenyl group.
 7. Theelectric cable according to claim 1, wherein the compound of formula (I)is diphenylethane, diphenylmethane or1,2,3,4-tetrahydro(1-phenylethyl)naphthalene.
 8. The electric cableaccording to claim 1, wherein the dielectric liquid comprises at least50% by mass of at least one compound of formula (I), relative to thetotal mass of the dielectric liquid.
 9. The electric cable according toclaim 1, wherein the ratio of the number of aromatic carbon atoms to thetotal number of carbon atoms in the dielectric liquid is greater than orequal to 0.6.
 10. The electric cable according to claim 1, wherein thedielectric liquid represents from 1% to 20% by mass relative to thetotal mass of the polymer composition.
 11. The electric cable accordingto claim 1, wherein the polypropylene-based thermoplastic polymermaterial comprises at least one homopolymer or one copolymer ofpropylene P₁, and at least one homopolymer or one copolymer of □-olefinP₂.
 12. The electric cable according to claim 11, wherein the propylenecopolymer P₁ is a copolymer of propylene and ethylene.
 13. The electriccable according to claim 11, wherein the propylene homopolymer orcopolymer P₁ represents from 40% to 70% by mass of thepolypropylene-based thermoplastic polymer material.
 14. The electriccable according to claim 11, wherein the □-olefin homopolymer orcopolymer P₂ is a heterophasic copolymer comprising a thermoplasticphase of propylene type and a thermoplastic elastomer phase of the typecopolymer of ethylene and of an □-olefin, a polyethylene or a mixturethereof.
 15. The electric cable according to claim 11, wherein the□-olefin homopolymer or copolymer P₂ represents from 30% to 60% by massof the thermoplastic polymer material.
 16. A process for preparing thepolymer composition of the at least one electrically insulating layer ofthe electric cable as defined in claim 1, further comprising at leastone step i) of mixing a polypropylene-based thermoplastic polymermaterial with said dielectric liquid.
 17. The electric cable accordingto claim 1, wherein the electrically insulating layer is anon-crosslinked layer.
 18. The process for manufacturing an electriccable as defined in claim 1 said process comprising at least one step 1)of extruding the polymer composition around an elongated electricallyconducting element, to obtain an electrically insulating layersurrounding said elongated electrically conducting element.